In a focal plane array of differential photodetectors, such as an array suitable for edge detection or targeting systems, the individual detector responsivities may vary by 10 percent or more between detectors. This variation in responsivity presents a problem, especially in large background applications in the presence of a relatively dim target, in that the difference signal delivered to an off-focal plane processor contains a background component having a magnitude related to the difference in responsivities between the two photodetectors. This background component can be compensated for by off-focal plane processors or by restricting the upper end of the dynamic range to approximately the value of the difference in responsivity between the two photodetectors; for example approximately 10 percent. However, in relation to the former compensation method off-focal plane processors are expensive. Also, for those applications wherein package size and power consumption are important considerations, such as in relatively small guided missiles, additional off-focal plane processors are generally undesirable. In relation to the latter compensation method restriction of the dynamic range may be undesirable when increased sensitivity is required in that an increase in sensitivity typically dictates a smaller upper end dynamic range.
Alternate methods of providing responsivity difference compensation for two photodetectors are depicted in FIGS. 1 and 2. In FIG. 1 two differential detectors are connected in an anode to cathode configuration and provide a signal to a single amplifier such as, for example, a reset integrator-type bilateral transimpedance amplifier which integrates both positively and negatively. However, the difference in responsivities must be compensated for in an off-focal plane processor wherein the dynamic range may be consumed entirely by the responsivity variations. Another, more fundamental, problem with this technique is that an integrated array of such differential detectors is not readily fabricated using conventional array fabrication methodologies in that conventional fabrication permits, within an array, either all cathodes or all anodes of photodetectors to be coupled together to a common node.
In FIG. 2 there is shown a separate transimpedance amplifier for each detector, the amplifier outputs being coupled to an off-focal plane differencing and responsivity compensating processor. This technique, although readily achievable by present processing techniques, requires that the entire dynamic range be defined by the maximum background level, resulting in a significant decrease in sensitivity. Furthermore, no background suppression occurs at the focal plane, placing a larger signal processing burden on the off-focal plane electronics.
It is therefore one object of the invention to provide a circuit to differentially couple together two photodetectors to an amplifier such that a resulting differential output current is independent of the intrinsic responsivity difference between the two photodetectors.
It is a further object of the invention to provide a circuit to differentially couple together two photodetectors to an amplifier such that a resulting output current is proportional to the difference in radiation flux received by each of the photodetectors.
It is a further object of the invention to provide a circuit to differentially couple together two photodetectors to an amplifier such that a resulting output current is proportional to the difference in radiation flux received by each of the photodetectors, the circuit being useable with any known type of current-mode photodetector, including both photo-voltaic and photo-conductive detectors.
It is another object of the invention to provide a circuit to differentially couple together two photodetectors to an amplifier such that a resulting differential output current is independent of the intrinsic responsivity difference between the two photodetectors, the circuit also providing an autocalibration capability.
It is still another object of the invention to provide a circuit to differentially couple together two photodetectors to a transimpedance amplifier, the circuit including a pair of MOSFET transistors operated in a subthreshold region and a capacitor which stores a logarithmic representation of a ratio of detector responsivities and also a representation of a threshold difference between the MOSFET transistors.
It is one still further object of the invention to provide a circuit to differentially couple together two photodetectors to a transimpedance amplifier, the circuit including a pair of MOSFET transistors operated in a subthreshold region and a capacitor, and to a method of calibrating the circuit which includes uniformly illuminating both detectors with a reference flux such that the capacitor stores a logarithmic representation of a ratio of detector responsivities and also a representation of a threshold difference between the MOSFET transistors.